That's an awfully expensive experiment
The concept doesn't make any sense to me, but if you think it still has hope, then keep at it.
Reminds me of the Virzi plate used in some violins and things. Hanging it from the CF grid by 3 points like a Virzi would be a good thing to try. Maybe do some chladni testing to find good locations for the anchors. Glue 3 little wood blocks to the free plate and to your bench or something sturdy, sprinkle glitter or something on the plate, and play various frequency sine waves through a speaker to get it vibrating and see what kind of mode shapes you get for various anchor layouts.
One thing to keep in mind is that any bracing should be done equally on both sides of the soundboard. If you use a wood plate with unconstrained edges, braced on one side only, it will potato chip quite dramatically when the humidity drops, and would need a lot of clearance to avoid contacting your CF grid. Not as dramatically with your western redcedar as it would be with spruce, but still significant.
But without any tension on the soundboard, you could leave it completely unbraced, and just use thickness to change the vibrational behavior. Doesn't have to be the same thickness all over
My advice: Don't spend much more money on the project until you're sure it will work. Do lots of experiments, but keep them cheap or free. It doesn't seem to me like it would sound good, but most builders say a rosewood top wouldn't sound good, and that's what I'm working on... Always fun to try new things